1. Field of the Invention
This invention relates to an electric power supply unit for providing a predetermined output voltage through conversion of a power supply voltage of a dc power supply such as a battery, and to a portable device equipped with such power supply unit.
2. Description of the Related Art
A power supply unit such as a series regulator typically converts the power supply voltage of a dc power supply to a predetermined output voltage. It is necessary for such power supply unit to suppress inrush current that would otherwise flow into a load and a smoothing capacitor connected to the power supply unit.
A conventional power supply unit can be provided with a capacitor connected in series with a constant current source and a power switch connected in parallel with the capacitor such that when the power switch is turned off (or opened) the constant current can gradually charge the capacitor, thereby causing the power supply unit to undergo a slow start, as disclosed in Japanese Patent Application Laid Open H7-336999. In this case, the number of external terminals of the power supply unit can be reduced since the power switch can control the operation of the power supply unit.
However, this type of conventional power supply circuit requires a predetermined current from a constant current source to flow through the power switch even in the standby condition (where the power switch is turned on) to ensure that the capacitor can be charged if the power switch is turned off.
Thus, the conventional power supply unit inevitably consumes a certain amount of current in the standby condition. In order to extend the continuously operable time of a battery powered portable device, minimization of its consumption current is strongly desired. Therefore, in order to perform soft start, it is not desirable that the current increases.